The present invention relates to an electron source apparatus having a plurality of electron-emitting devices wired in a matrix, and an image forming apparatus using the electron source apparatus.
Conventionally, two types of devices, namely thermionic and cold cathode devices, are known as electron-emitting devices. Known examples of the cold cathode devices are field emission type electron-emitting devices (to be referred to as FE type electron-emitting devices hereinafter), and metal/insulator/metal type electron-emitting devices (to be referred to as MIM type electron-emitting devices hereinafter). A known example of surface-conduction emission type electron-emitting devices is described in, e.g., M. I. Elinson, xe2x80x9cRadio Eng. Electron Phys., 10, 1290 (1965) and other examples will be described later.
The surface-conduction emission type electron-emitting device utilizes the phenomenon that electrons are emitted by a small-area thin film formed on a substrate by flowing a current in parallel with the film surface. The surface-conduction emission type electron-emitting device includes electron-emitting devices using an Au thin film [G. Dittmer, xe2x80x9cThin Solid Filmsxe2x80x9d, 9, 317 (1972)], an In2O3/SnO2 thin film[ M. Hartwell and C. G. Fonstad, xe2x80x9cIEEE Trans. ED Conf.xe2x80x9d, 519 (1975)] , a carbon thin film [Hisashi Araki et al., xe2x80x9cVacuumxe2x80x9d, Vol. 26, No. 1, p. 22 (1983)], and the like, in addition to an SnO2 thin film according to Elinson mentioned above.
Known examples of the FE type electron-emitting devices are described in W. P. Dyke and W. W. Dolan, xe2x80x9cField emissionxe2x80x9d, Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, xe2x80x9cPhysical properties of thin-film field emission cathodes with molybdenium conesxe2x80x9d, J. Appl. Phys., 47, 5248 (1976).
As another FE type device structure, there is an example in which an emitter and gate electrode are arranged on a substrate to be almost parallel to the surface of the substrate.
A known example of the MIM type electron-emitting devices is described in C. A. Mead, xe2x80x9cOperation of tunnel emission Devicesxe2x80x9d, J. Appl. Phys., 32,646 (1961).
Since the above-described cold cathode devices can emit electrons at a temperature lower than that for thermionic cathode devices, they do not require any heater. The cold cathode device has a structure simpler than that of the thermionic cathode device and can shrink in feature size. Even if a large number of devices are arranged on a substrate at a high density, problems such as heat fusion of the substrate hardly arise. In addition, the response speed of the cold cathode device is high, while the response speed of the thermionic cathode device is low because thermionic cathode device operates upon heating by a heater.
For this reason, applications of the cold cathode devices have enthusiastically been studied.
Of cold cathode devices, the surface-conduction emission type electron-emitting devices have a simple structure and can be easily manufactured, so that many devices can be formed on a wide area. As disclosed in Japanese Patent Laid-Open No. 64-31332 filed by the present applicant, a method of arranging and driving many devices has been studied.
Regarding applications of the surface-conduction emission type electron-emitting devices, e.g., image forming apparatuses such as an image display apparatus and image recording apparatus, charge beam sources, and the like have been studied.
Particularly as an application to image display apparatuses, as disclosed in the U.S. Pat. No. 5,066,883 and Japanese Patent Laid-Open Nos. 2-257551 and 4-28137 filed by the present applicant, an image display apparatus using a combination of a surface-conduction emission type electron-emitting device and a fluorescent substance which emits light upon irradiation with an electron beam has been studied. This type of image display apparatus using a combination of the surface-conduction emission type electron-emitting device and fluorescent substance is expected to exhibit more excellent characteristics than other conventional image display apparatuses. For example, compared to recent popular liquid crystal display apparatuses, the above display apparatus is superior in that it does not require any backlight because of self-emission type and that it has a wide view angle.
A method of driving a plurality of FE type electron-emitting devices arranged side by side is disclosed in, e.g., U.S. Pat. No. 4,904,895 filed by the present applicant. A known application of FE type electron-emitting devices to an image display apparatus is a flat panel display apparatus reported by R. Meyer et al. [R. Meyer: xe2x80x9cRecent Development on Microtips Display at LETIxe2x80x9d, Tech. Digest of 4th Int. Vacuum Microelectronics Conf., Nagahara, pp. 6-9 (1991)]. An application of many MIM type electron-emitting devices arranged side by side to an image display apparatus is disclosed in Japanese Patent Laid-Open No. 3-55738 filed by the present applicant.
FIG. 1 shows an example of a multi electron source wiring method. In the electron source shown in FIG. 1, m cold cathode devices in the vertical direction and n cold cathode devices in the horizontal direction, i.e., a total of nxc3x97m cold cathode devices are two-dimensionally arrayed in a matrix. In FIG. 1, reference numeral 3074 denotes a cold cathode device; 3072, a row-direction wiring line; 3073, a column-direction wiring line; 3075, a wiring resistance of the row-direction wiring line; and 3076, a wiring resistance of the column-direction wiring line. Reference symbols Dx1, Dx2, . . . , Dxm denote feeding terminals of the row-direction wiring lines; and Dy1, Dy2, . . . , Dyn, feeding terminals of the column-direction wiring lines. This simple wiring method is called a matrix wiring method. The matrix wiring method can easily manufacture a multi electron source because of a simple structure.
When a multi electron beam by the matrix wiring method is to be applied to an image forming apparatus, m and n must be several hundreds or more in order to ensure the display capacitance. Further, a cold cathode device must accurately output an electron beam with a desired intensity in order to display an image at an accurate luminance.
When many cold cathode devices wired in a matrix are to be driven, devices of one row of the matrix are simultaneously driven. The row to be driven is sequentially switched to scan all the rows. According to this method, the driving time assigned each device is ensured n times longer than in a method of sequentially scanning all the devices one by one. Thus, the luminance of the display apparatus can be increased.
More specifically, there are proposed an arrangement in which a voltage source is connected to matrix wiring to drive devices, and a method of driving FE type devices using a controlled constant current source, as disclosed in U.S. Pat. No. 5,300,862 to Parker et al. FIG. 2 is a circuit diagram for explaining this.
In U.S. Pat. No. 5,300,862, the X direction shown in FIG. 2 is a row direction, and the Y direction is a column direction. In the following description, however, the X direction is defined as a column direction, and the Y direction is defined as a row direction in order to match the description of the present invention.
In FIG. 2, reference numerals 2201a, 2201b, and 2201c denote controlled constant current sources; 2202, a switching circuit; 2203, a voltage source; 2204a, column wiring lines; 2204b, row wiring lines; and 2205, FE type devices.
The switching circuit 2202 selects one of the row wiring lines 2204b, and connects it to the voltage source 2203. The controlled constant current sources 2201a, 2201b, and 2201c supply currents to the respective column wiring lines 2204a. These operations are properly performed in synchronism with each other to drive FE type devices of one row.
Arrangements in which an electron source having surface-conduction emission type electron-emitting devices is driven using a constant current source are disclosed in European Patent Laid-Open EP688035A, EP762371A, EP762372A, and EP798691A.
The characteristics of the cold cathode electron-emitting device described above are influenced more or less by an atmosphere (vacuum degree or the quality of vacuum) in which the device is arranged. When the electron-emitting device is driven, various gases are discharged by the device itself and a member irradiated with an electron beam emitted by the device. If such gas is discharged, it influences not only the characteristics of each device but also the characteristics of an adjacent device in an electron source or image forming apparatus in which many electron-emitting devices must be arrayed at a high density. To prevent this, important is how to keep high vacuum in the atmosphere where electron-emitting devices are arranged in the electron source or image forming apparatus in which electron-emitting devices are arrayed at a high density.
As a solution, a getter for exhausting a gas is considered to be arranged near each electron-emitting device. There are proposed an arrangement (see Japanese Patent Laid-Open No. 9-82245) in which the getter is arranged on a wiring line for driving each electron-emitting device, and an arrangement (see Japanese Patent Laid-Open No. 4-12436) in which the wiring line itself is formed from the getter.
An atmosphere around each electron-emitting device can be kept high vacuum by arranging a getter. In particular, it is preferable that the getter be directly arranged on the wiring line, or the wiring line itself be formed from the getter, as described above.
The getter exhausts a gas present around it by chemically or physically adsorbing the gas present in the atmosphere to its surface. As the getter exhausts a larger amount of gas present in the atmosphere, the composition of the getter itself changes over time. From this, the inventor of the present application has found that changes in getter itself over time lead to changes in the resistance of an electrical path extending from a driving circuit to an electron-emitting device in an arrangement in which the getter material is electrically connected to a wiring line such that the getter is arranged on the wiring line or the wiring line itself is formed from the getter material, as described above.
The degree of changes in the composition of the getter itself changes depending on a position where the getter is arranged or the driving state of an adjacent electron-emitting device. The inventor of the present application has found that even if a desired state is ensured in the initial stage such that the resistance values of electrical paths extending from the driving circuit to respective electron-emitting devices are uniform, the resistances of the electrical paths vary with the lapse of time.
According to the finding of the inventor of the present application, the influence of changes in getter over time typically appears when the getter is arranged on a row-direction wiring line in an arrangement which has electron-emitting devices arranged in a matrix, and sequentially scans row-direction wiring lines to line-sequentially drive the devices. In addition, the influence especially typically appears when the getter is arranged on a row-direction wiring line in an arrangement using an electron-emitting device in which a current flowing into the electron-emitting device is equal to or larger than an emitted current. The influence typically appears when the getter is electrically connected to a wiring line such that the getter is in contact with the wiring line, and particularly typically appears when the wiring line is a row-direction wiring line as a scanning wiring line, or when the electron-emitting device is an electron-emitting device in which a current flowing into the electron-emitting device is equal to or larger than an emitted current.
From these results, the inventor of the present application has found that if the electron-emitting device is driven in accordance with electron emission characteristics and/or the resistance of the electrical path in the initial state, uniformity of the electron emission characteristics of the electron source may degrade, or variations in the luminance of the display or color misregistration may occur.
The inventor of the present application has found the influence of the getter on the electrical path extending from the driving circuit to the electron-emitting device, and has found as a result of extensive studies an arrangement capable of suitably driving the electron-emitting device even with an arrangement suffering this influence.
More specifically, an invention according to the present application can implement an electron source and image forming apparatus which have long service lives, almost no characteristic variations, and high uniformity.
One invention of an electron source according to the present application has the following arrangement.
An electron source comprises:
an electron source substrate which has on a substrate a plurality of row-direction wiring lines, a plurality of column-direction wiring lines, insulating layers formed at intersections between the row-direction wiring lines and the column-direction wiring lines, a plurality of electron-emitting devices connected to the row-direction wiring lines and the column-direction wiring lines, and getters arranged on the wiring lines;
a circuit for sequentially applying a selection potential to the plurality of row-direction wiring lines; and
a controlled constant current application circuit for applying a controlled current to the plurality of column-direction wiring lines.
This invention is especially effective in an arrangement in which the getter is arranged on the row-direction wiring line.
Another invention of an electron source according to the present application has the following arrangement.
An electron source comprises:
an electron source substrate which has on a substrate a plurality of row-direction wiring lines, a plurality of column-direction wiring lines, insulating layers formed at intersections between the row-direction wiring lines and the column-direction wiring lines, and a plurality of electron-emitting devices connected to the row-direction wiring lines and the column-direction wiring lines, the wiring lines being electrically connected to getters other than the electron-emitting devices;
a circuit for sequentially applying a selection potential to the plurality of row-direction wiring lines; and
a controlled constant current application circuit for applying a controlled current to the plurality of column-direction wiring lines.
This invention is especially effective in an arrangement in which the getter is arranged to be electrically connected to the row-direction wiring line.
Each invention described above is particularly effective when the electron-emitting device is an electron-emitting device in which a current flowing into the electron-emitting device is larger than a current emitted by the electron-emitting device. For example, the arrangement of each invention is particularly effective for a surface-conduction emission type electron-emitting device since a current flowing into the electron-emitting device is much larger than an emitted current.
The getter in each invention described above has characteristics of adsorbing substances in the atmosphere. The getter is preferably a metal or alloy containing at least any one of Ti, Zr, Hf, V, Nb, Ta, and W.
Since the electron source of each invention described above has a getter, a gas discharged by the electron-emitting device itself or a gas discharged by a member irradiated with an emitted electron beam are rapidly exhausted. By electrically connecting the getter to a wiring line such that the getter is arranged on the wiring line, the potential of the getter is prevented from becoming unstable. Even if the resistance value of an electrical path extending from a driving circuit to an electron-emitting device varies owing to changes in getter over time, the controlled constant current application circuit is used to flow a predetermined current regardless of the resistance value of the electrical path. This suppresses variations in voltages applied to respective devices. As a result, an electron source having a long service life, almost no characteristic variations, and high uniformity can be attained.
The selection potential applied to the row-direction wiring line in each invention is a potential at which an electron-emitting device connected to a row-direction wiring line which receives the selection potential can emit electrons in cooperation with control from a column-direction wiring line. The selection potential is sequentially applied to row-direction wiring lines to line-sequentially drive devices.
The circuit for sequentially applying the selection potential to row-direction wiring lines and the controlled current application circuit in each invention can adopt various arrangements, and can be implemented as an integrated circuit.
In each invention, an arrangement in which the row-direction wiring lines are arranged on the column-direction wiring lines via insulating layers is preferable.
An image forming apparatus having an electron source and a substrate which has an image forming member for forming an image by irradiation of electrons from the electron source and is arranged to face the electron source can preferably employ an arrangement using the electron source of each invention as the electron source. As the image forming member, fluorescent substances can be suitably adopted. According to the present application, an image forming apparatus having a long service life, almost no characteristic variations, and high uniformity can be attained.